1. Field of the Invention
The present invention relates to a communication system for performing a mode selection by selecting or negotiating the mode to be used, a method to be performed in such a communication system, and to a network element capable of mode selection.
2. Description of the Prior Art
Communication networks transfer information such as user voice traffic or the like, on a packet-switched and/or circuit-switched basis using modes which may be commanded by the system or negotiated between the involved network elements such as end user equipments. As an example, in planned evolution of networks such as UMTS (Universal Mobile Telecommunication System) systems, additional functions and services can be incorporated. For instance, novel multimedia services, such as multimedia messaging services MMS, are supported within the system which services are IP (Internet Protocol)-based services. Packet-based (e.g. IP-based) service sessions such as multimedia service sessions may be controlled by a specific protocol. As an example, the Session Initiation Protocol (SIP) represents a protocol which may be used for example for call and connection establishment as well as for transport of endpoint capability information. Such capability information may for example relate to voice and multimedia codecs supported by the end terminals.
The functionality and services of such multimedia service systems are mapped onto the existing network system functions, for example of UMTS type. As an example, the system services may be mapped to the PDP contexts and radio signalling, as well as to existing packet-switched core network elements and interfaces, for example of a UMTS type. Hence, there is a problem of multimedia (e.g. IP multimedia) and network layer (e.g. GPRS layer) interactions and mapping.
As an example, in case of VoIP calls (voice over IP-based connection, that is Internet telephony), the radio access network such as GERAN (“GSM/Edge Radio Access Network”) and UTRAN (UMTS Terrestrial Radio Access Network), may be informed of the type of application for deciding of the header adaptation method to be used for example a particular PDP context. As an example, two different header adaptation schemes available for selection can for example be “header compression” and “header stripping/removal”. The header stripping/removal mode may be used for speech-only traffic where for example optimized speech transport is required for instance for integrated lower-end terminal devices. A header compression mode may be utilized for example for more general IP multimedia traffic including voice application operation on an external device such as a laptop computer connected to a UMTS phone.
When an inappropriate mode such as inappropriate protocol mode, header adaptation mode or radio access bearer mode are selected, problems in incorrect message transmission may occur.